Conventional ethylene homopolymers or copolymers with α-olefin have been typically prepared using so-called a Ziegler-Natta catalytic system comprising a titanium or vanadium compound as a main catalyst and an alkylaluminum compound as a co-catalyst. Although the Ziegler-Natta catalytic system is highly active for ethylene polymerization, it has non-uniform active sites, so that the produced polymer has a wide molecular weight distribution, and, in particular, the composition distribution is not uniform in copolymerization of ethylene and α-olefin.
Recently, there has been developed a metallocene catalytic system composed of a metallocene compound of Group 4 transition metal of the periodic table, such as titanium, zirconium, hafnium, etc., and a co-catalyst such as methylaluminoxane. Because the metallocene catalytic system is a homogeneous catalyst having single active sites, it enables the preparation of polyethylene having a narrower molecular weight distribution and a more uniform composition distribution, compared to when using the conventional Ziegler-Natta catalytic system. For example, EP Patent Application Publication Nos. 320,762 and 3,726,325, Japanese Patent Laid-open Publication No. Sho. 63-092621, and Japanese Patent Laid-open Publication Nos. Hei. 02-84405 and 03-2347 disclose a metallocene compound such as Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, ethylene(IndH4)2ZrC12, etc., which is activated with a methylaluminoxane co-catalyst, so that ethylene is highly actively polymerized, thereby preparing polyethylene having a molecular weight distribution (Mw/Mn) of 1.5˜2.0. However, this catalytic system makes it difficult to obtain a high-molecular-weight polymer. In particular, when this is applied to solution polymerization at a high temperature of at least 140° C., polymerization activity is drastically decreased and (β-dehydrogenation is predominantly carried out, and thus such a catalytic system is known to be unsuitable to prepare a high-molecular-weight polymer having a weight average molecular weight (Mw) of 100,000 or more.
U.S. Pat. No. 5,084,534 by Exxon discloses the preparation of a copolymer having a narrow molecular weight distribution of 1.8˜3.0 and a uniform composition distribution by polymerizing ethylene alone or ethylene with 1-hexene or 1-octene at 150˜200° C. using a (n-BuCp)2ZrCl2 catalyst and a methylaluminoxane co-catalyst. In addition, EP Patent Nos. 0416815 and 0420436, by Dow, disclose a catalyst the structure of which is geometrically controlled by connecting an amide group in the form of a ring to a cyclopentadiene ligand, and which exhibits high catalytic activity upon polymerizing ethylene alone or ethylene with α-olefin under slurry polymerization or solution polymerization conditions and also increases high reactivity with comonomers, thereby enabling the preparation of a high-molecular-weight polymer having a uniform composition distribution. As in the metallocene catalyst, however, the above catalyst is drastically deteriorated in terms of catalytic stability and comonomer incorporations in proportion to an increase in the temperature under high-temperature solution polymerization conditions of at least 140° C., and economic benefits negate attributed to high material cost, making it difficult to industrially use it.